There are known methods of producing a phthalic dichloride compound; a method of chlorinating ortho-xylene and then hydrolyzing the resultant, and a method of bringing a phthalic anhydride compound in reaction with a chlorinating agent.
Examples of the method of using ortho-xylene as a starting material include a method in which 1-dichloromethyl-2-(trichloromethyl)benzene is produced by photo-chlorination of ortho-xylene, and then hydrolyzed to produce 3-chlorophthalide, and then the produced 3-chlorophthalide is subjected to photo-chlorination (refer to Patent Literature 1). However, this method needs equipments for photo-chlorination and requires multistep-production processes using a massive amount of chlorine.
Meanwhile, examples of the method of using a phthalic anhydride compound as a starting material include the following method. For example, a method in which phthalic anhydride is reacted with phosphorous pentachloride is disclosed (refer to Non-Patent Literature 1). However, this method produces waste containing a massive amount of phosphorus.
Further, examples of the method include a method in which a phthalic anhydride compound is reacted with phosgene or thionyl chloride (refer to Patent Literatures 2 to 5). In the method, special production equipment is needed because toxic and dangerous phosgene is to be handled. Besides, in the case of using thionyl chloride, equipment is needed for reaction of thionyl chloride under high-pressure.
In addition, examples of the method include a method in which phthalic anhydride is reacted with (trichloromethyl)benzene in the presence of zinc chloride (refer to Non-Patent Literature 2 and Patent Literatures 6 and 7). This method is a convenient method in which the reaction is carried out in an ordinary reactor (oven). However, it is necessary to use a massive amount of zinc chloride depending on a reaction temperature. According to the Non-Patent Literature 2, for example, it is reported that when zinc chloride is used in an amount of 10 mole % with respect to phthalic anhydride, the reaction proceeds at a temperature of from 110° C. to 120° C. Such a great amount of catalyst is however not practical. In contrast, if the amount of zinc chloride is reduced to 1 mole % which is still a large quantity, such high temperature as 200° C. is needed to achieve a practical reaction speed. It is difficult for an ordinary aqueous medium to act as a heating medium for realizing such high reaction temperature. On instead, a special medium is needed. Further, special equipments for addressing this demand must be provided.    Patent Literature 1: JP-A-47-27949 public patent bulletin    Patent Literature 2: International Publication (WO) No. 2006/056436 pamphlet    Patent Literature 3: JP-A-2005-330283 public patent bulletin    Patent Literature 4: US Patent Application Publication No. 2007/299282 specification    Patent Literature 5: International Publication (WO) No. 2006/058642 pamphlet    Patent Literature 6: U.S. Pat. No. 1,963,748 specification    Patent Literature 7: U.S. Pat. No. 1,963,749 specification            Non-Patent Literature 1: Org. Synth. Coll. Vol II, page 528        Non-Patent Literature 2: J. Am. Chem. Soc., Vol. 59, page 206 (1937)        